Modal Operator double keytrigger (RELEASE triggered twice on Ubuntu)
The problem here was with the operating system /environment, and confirmed by pink-vertex. While this isn't a bug report, it may still be handy to keep around in case others experience it.
The behaviour had me stumped until I wrote the above code to localize what was happening.
This seems to be a long-standing bug in the windowing system that ubuntu uses: https://developer.blender.org/T21528 (dating:  Mar 9 2010)

knowing it's broken isn't good enough.
@NoamPeled asks how one might get a working solution in the case of the windowing system throwing multiple repeat event.values, like RELEASE -- thus  increaseing the potential for the interface to behave in unexpected ways (ie a quick tap of a key could trigger release once or twice, but mostly twice -- at the very least the lack of consistency is the true issue then)
untested and verbose,
# somewhere at the top of your python file
# add any other keys you want to track
event_values = {'MINUS': None} 

# your modal operator code (snipped for brevity, see above in the question)

    def modal(self, context, event):

        # somewhere it makes sense
        if event.type in event_values.keys():
            if event.value == event_values.get(event.type):
                pass  # or some other passthrough/ignore
            else:
                event_values[event.type] = event.value
                # do some other useful thing